The features of the present invention may be used in the printing arts and more particularly in electrophotographic printing. In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image in a region known as the development zone. Toner paritcles are attracted from carrier granules or bead carriers of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a copy sheet and permanently affixed thereto. The foregoing generally describes a typical mono-color electrophotographic copying machine.
Recently, electrophotographic printing machines have been developed which produce highlight color copies. A typical highlight color printing machine records successive electrostatic latent images on the photoconductive surface. When combined, thes electrostatic latent images form a total latent image corresponding to the entire original document being reproduced.
One latent image is usually developed with black toner particles. The other latent image is developed with color highlighting toner particles, e.g. red toner particles. These developed toner images are transferred sequentially to the copy sheet to form the color highlighted copy. A color highlight printing machine of this type is a two pass machine.
Single pass highlight color printing machines using tri-level printing have also been developed. Tri-level electrophotographic printing is described in detail in U.S. Pat. No. 4,078,929. As described in this patent, the latent image is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level electrophotographic printing, the charge on the photoconductive surface is divided in three, rather than two, ways as is the case in mono-color printing. The photoconductive surface is charged, typically to about 900 volts. It is exposed image wise, such that one image corresponding to charged image areas remains at the full potential of 900 volts. The other image, which corresponds to discharged image areas is exposed to discharge the photoconductive surface to its residual potential of typically about 100 volts. The background areas are exposed to reduce the photoconductive surface potential to about halfway between the charged and discharged potentials, (typically about 500 volts). The developer unit arranged to develop the charged image areas, is typically biased to about 600 volts, and the developer unit, arranged to develop the discharged image areas, is biased to about 400 volts.
The single pass nature of this system dictates that the electrostatic latent image pass through the developer units in a serial fashion. When the latent image, which has a high charged image potential region and a low charge image potential region, passes through the first developer unit, arranged to develop the discharged image areas, an extremely high cleaning field potential is established between the electrically biased developer unit and the highly charge image areas of the latent image. This high cleaning field potential attracts developer material from the developer unit onto the highly charged image areas. When this occurs, the highly charged image areas of the electrostatic latent image are locally discharged where developed, and as a result, white spots will be noticeable in the solid area images developed by the second developer unit, which at the present time is black, rendering the prints unacceptable.
This problem was overcome by the invention disclosed in the below-referenced co-pending U.S. application Ser. No. 07/604,269. However, electrostatic forces and adhesion forces within the developer units contribute to a condition where the bead carriers are carried out of the development unit. Bead removal devices (BRDs) are well known and commonly used to pick off any developer carriers which are carried out of the development zone of a development unit.
Generally BRDs operate by generating a strong magnetic field in the area between the photoconductive surface and the BRD to attract free bead carriers to the shell of the BRD. These captured beads are then deposited in a sump or developer receiver as the shell of the BRD is rotated. This arrangement, however, renders release of beads from the BRD more difficult, e.g. gravity and centripetal forces often are insufficient to achieve release of the beads from the magnetic field as the shell of the BRD rotates. That is the strong magnetic field necessary to attract bead carriers from the photoconductive surface to the BRD shell are sufficiently strong around the shell itself to retain some bead carriers as the shell rotates.
As disclosed in U.S. Pat. No. 4,829,338, the magnetic field from the magnet positioned in the shell of the BRD can be directed by use of a ferromagnetic shunt to promote bead removal from the photoconductive surface while enhancing the field between the photoconductive surface and the BRD to attract free beads. Nevertheless, this solution has not overcome all problems associated with BRDs.
Specifically, some bead carriers remain attached to the shell as the shell rotates. These unreleased beads tend to attract additional beads to themselves to form bead chains. These bead chains can span the gap between the BRD and photoconductive surface causing degradation of the image being reproduced. For example, in highlight color electrophotographic printing machines, when bead chains occur in the first discharged developer unit, portions of the second latent image which are contacted by the chains are discharged thereby causing discharge line defects in the finished print.
Also, the arrangement disclosed in U.S. Pat. No. 4,829,338 does not address carrier pickup and carry through in reverse rotation of a BRD. In highlight color electrographic printing machines using magnetic delivery means, monochromatic images are often achieved by disabling one of the developer units. One technique is to reverse the direction of the magnetic brush rollers as disclosed in U.S. Pat. No. 4,811,046 to May, which is incorporated by reference herein. The reversal of the direction of travel of the rollers in the developing units in this case effects a substantial reduction in developer available to effect the charged photoconductive surface. Such reversal of angular travel of the rollers can also be done during warm-up and shut down cycles to remove stray or extraneous developer materials from the developer zone. However, reversed rotation of the rollers is not entirely satisfactory, as some beads are not released and certain other beads are attracted to and attach to the roller. Failure to release the beads, as well as the attraction of additional beads, results, for example, in discharge of portions of the latent image on the photoconductive surface as it passes through a developer unit to a secondary developer unit.
Another problem associated with the use of magnetic carrier beads and toner to form the developer material is the failure to remove substantially all of the beads that are attracted to the photoconductive surface. This problem seems to increase as the speed of the latent image bearing surface increases. Thus, in the effort to improve the output of devices by increasing the speed of the device, it seems that known means for removing excess and unwanted materials from the surface are insufficient. For example, it appears that beads which are smaller than average size or which are fragments or fractured portions of full sized beads tend to be more susceptible to carry past the developer station to the transfer and fusing stations. In any event, as previously mentioned, bead carry through is a significant problem which has deleterious effects upon the quality of prints made by electrophotographic printers employing two part magnetic toner.
Yet another problem associated with the developer rollers is that during ordinary operation some beads fail to release from the developer rollers. When this involves a substantial number of beads, a condition mimicking a low toner condition results. Essentially, empty beads are being transported through the development zone and insufficient toner is delivered to develop the latent image. Optimally, the magnetic field at the release point the of development rollers must be minimized to reduce carry through of `empty` beads and to prevent carry back of beads in the "reversed" rotating rollers. Further, this must be accomplished without substantially affecting the magnetic field characteristics along the remainder of the rollers which are responsible for bead pickup, bead transport, bead trimming, developing by toner, migration from bead to photoconductive surface and carryout of beads from development zone. Also, the magnetic fields around a BRD housing must be maximized along the interface between the BRD shell and the photoconductive surface and minimized at all other points to afford return of released beads to a sump or reservoir and to impede bead chain formation.
Another vexing problem occurs with electrically biasing the BRD with an alternating current to improve bead attraction. Specifically, the BRD at higher bias levels becomes so attractive that the attracted beads and excess toner fail to release at desired locations. Rather, the formation of bead chains is encouraged, and the aggregation of materials on the BRD severely degrades its performance so that beads and free toner are not attracted to the BRD.
Various techniques have heretobefore been used to develop electrostatic latent images as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
U.S. Pat. No. 4,320,958; Patentee: Fantuzzo; Issued: Mar. 23, 1982.
U.S. Pat. No. 4,641,946; Patentee: Forbes II; Issued: Feb. 10, 1987.
U.S. Pat. No. 4,833,504; Patentee: Parker et al.; Issued: May 23, 1989.
Co-pending U.S. application Ser. No. 07/604,269; Applicant: Hogestyn; Filed: Oct. 29, 1990.
U.S. Pat. No. 4,292,924; Patentee: Lindblad et al.; Issued: Oct. 6, 1981.
U.S. Pat. No. 5,063,412; Patentee: Hirsch; Issued: Nov. 5, 1991.
U.S. Pat. No. 4,972,231; Patentee: Bares; Issued: Nov. 20, 1990.
U.S. Pat. No. 4,878,089; Patentee: Guslits et al; Issued: Oct. 31, 1989.
U.S. Pat. No. 4,466,730; Patentee: Jugle, et al.; Issued: Aug. 1, 1984.
U.S. Pat. No. 4,797,708; Patentee: Kasiske, Jr. et al.; Issued: Jan. 10, 1989.
U.S. Pat. No. 5,225,880; Patentee: Shehata et al.; Issued: Jul. 6, 1993.
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,320,958 discloses a processing station for an electrophotographic printing which cleans the photoconductive surface and develops an electrostatic latent image recorded thereon. The processing unit uses an indexable magnet positioned interiorly of a rotating tubular sleeve. During development, the magnet is indexed so that a weak magnetic field is generated in the development zone during development. During cleaning, the magnet is indexed to generate a strong magnetic field in the cleaning zone.
U.S. Pat. No. 4,641,946 describes a developer roller having a rotating tubular sleeve with a magnet disposed interiorly thereof. A photoconductive belt is wrapped about a portion of the exterior surface of the sleeve. The magnet generates a radial magnetic field in the development zone which, at the center, may range from 0 gauss to 500 gauss. FIG. 4 shows a radial magnetic field, in the development zone, having a valley of about -185 gauss and twin peaks, each of about -385 gauss.
U.S. Pat. No. 4,833,504 discloses a single pass highlight color electrophotographic printing machine using two developer units. The first developer unit contains developer with black toner. The black toner is driven to the most highly charged areas of the latent image by the electrostatic field between the photoreceptor and developer rolls. The second developer unit contains developer with the highlight color toner. The highlight color toner is urged towards the parts of the latent image at the residual potential, i.e. the discharged region of the latent image, by the electrostatic field between the photoreceptor and the development rolls in the second housing. The magnetic rolls in the second developer unit are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and subjects the image developed by the first developer unit to minimal disturbance. In addition, the developer is transported from one magnetic roll to the next.
Co-pending U.S. patent application Ser. No. 07/604,269 describes an electrophotographic printing machine in which an electrostatic latent image is recorded on a photoconductive surface. One portion of the latent image is a discharged area with the other portion of the latent image being a charged area. The discharged image area is developed with toner particles of a first color and polarity by a first developer unit. The first developer unit generates a weak magnetic field in the development zone and a strong magnetic field at the entrance and exit of the development zone. A second developer unit develops the charged image area with toner of a second color and polarity. The colors of the toners are different from one another.
U.S. Pat. No. 4,292,924 discloses a magnetic brush apparatus for cleaning photoconductive surfaces and suggests its application to magnetic brush development mechanisms. The disclosure device has a series of magnetics disposed within a rotating cylindrical sleeve disposed within a cleaning housing. The cleaning housing has an aperture which is proximate a photoconductive surface so that the surface is cleaned as it passes the aperture. The sleeve is provided with field shaping devices at the extreme ends of the cleaning roller so that developer material attracted to the sleeve does not build at the extreme ends of the cylindrical sleeve.
U.S. Pat. No. 5,063,412 discloses an electrophotographic printing machine with a rotating magnetic brush unit having magnetics disposed within the units. The magnetics disposed proximate the photoconductive surface are electromagnetic which are inactivated when developing a latent image and activated to exert a strong field when developer material is to be retained on the sleeve without developing the photoconductive surface.
U.S. Pat. No. 4,972,231 discloses a magnetic brush developer unit having a movable magnet disposed with the rotating portion of the magnetic brush developer. The magnet is movable between an active position away from the exterior wall of the unit and the photoconductive surface to permit development of latent images on the surface and an inactive position closer to the surface which impedes development of any latent images on the surface.
U.S. Pat. No. 4,878,089 discloses a magnetic brush developer station with a magnetic brush for applying developer material to a charge pattern bearing medium.
U.S. Pat. No. 4,466,730 discloses a magnetic brush developer unit having a housing in which a magnetic is disposed within a rotating tubular member for transporting carrier beads with toner adhereing thereto to a photoconductive surface. The area adjacent the tubular member and within the housing has a negative air pressure as air is withdraw from the housing so that airborne toner and toner on the metering blade are attracted to the bottom of the housing.
U.S. Pat. No. 4,797,708 discloses an electrophotographic printing machine with a vacuum slit or air knife positioned adjacent the photoreceptor for picking up large particles off the photoreceptor by aerodynamic drag as it passes within the vacuum flow.
U.S. Pat. No. 5,225,880 also discloses a system for removing agglomerates from a developed image on a photoreceptor in an electrophotographic printing machine.
In accordance with one aspect of the present invention, there is provided an apparatus for developing a latent image in an electrophotographic printing machine of the type having a latent image recorded on a moving charge retentive surface comprising magnetic means, generating a magnetic field, for attracting and moving magnetic carrier beads having toner particles adhering triboelectrically thereto to a development zone proximate to the photoconductive surface so that the latent image attracts toner particles from the carrier beads thereto. The apparatus of the present invention further includes magnetic pick off means positioned adjacent the surface and downstream from the development zone for removing magnetic carrier beads adhering to the surface and vacuum means for removing magnetic carrier beads adhering to said magnetic pick off means generating an aeriform fluid flow to remove magnetic carrier beads adhering to the magnetic pick off means.
In accordance with other aspects of the present invention, this provides an apparatus for developing a latent image recorded on a moving photoconductive surface comprising magnetic means, generating a magnetic field, for attracting and moving magnetic carrier beads having toner particles adhering triboelectrically thereto to a development zone proximate to the photoconductive surface so that the latent image attracts toner particles from the carrier beads thereto. This apparatus also includes combination vacuum and magnetic pick off means positioned adjacent the surface and downstream from the development zone for removing magnetic carrier beads adhering to the surface.
In accordance with yet another aspect of this invention, there is provided a method for developing a latent image recorded on a moving photoconductive surface comprising the steps of generating a magnetic field, for attracting and moving magnetic carrier beads having toner particles adhering triboelectrically thereto to a development zone proximate to the photoconductive surface so that the latent image attracts toner particles from the carrier beads thereto. The method also includes generating a second magnetic field with a magnetic pick off means positioned adjacent the surface and downstream from the development zone for removing magnetic carrier beads adhering to the surface and generating an aeriform fluid flow to remove magnetic carrier beads adhering to the magnetic pick off means.